In the fields of optical information communication, optical information processing, etc., nonlinear optical organic materials have been recently paid attention. The nonlinear laser materials mean materials showing such nonlinear optical effects that they convert a light of a laser into a light having different wavelength and they are varied in the refractive index under application of electric voltage. Particularly, second-order nonlinear optical organic materials having such effect that they convert a light of a laser into a light having a wavelength of 1/2 of the laser wavelength (second harmonic generation) have been paid much attention as materials far superior to inorganic materials since the nonlinear optical organic materials were reported (ACS Symposium Series 233, 1983). For example, KTiOPO.sub.4 (i.e., KTP) is known as the typical inorganic material, but development of a nonlinear optical organic material having higher nonlinear optical properties than KTP is expected.
The second-order nonlinear optical organic materials include those containing an aromatic ring, a donative substituent group and an accepting substituent group and having such a structure that .pi. electrons of the aromatic ring are polarized by the donative substituent group and the accepting substituent group in the molecule. The materials of this type is assumed to be extremely enhanced in the nonlinear optical response because nonlinear optical properties are produced on the .pi. electron sites.
However, para-nitroaniline which is expected to have high nonlinear optical response takes such a structure that the adjacent two molecules are inverted to each other when it is processed to be in the form of single crystal, this single crystal being essential for the nonlinear optical elements such as an optical wavelength conversion element and an electrooptical element in the practical use, resulting in losing of the nonlinear optical properties. That is, the nonlinear optical organic materials tend to lose the nonlinear optical properties when the adjacent molecules become to be centrally symmetric. For this reason, organic compounds have been synthesized in which a substituent group for getting rid of the symmetry of the adjacent molecules or a substituent group for imparting optical activity is introduced. For example, 2-methyl-4-nitroaniline synthesized as above has been confirmed to show high nonlinear optical properties.
The nonlinear optical organic materials tend to have higher nonlinear optical properties as the intramolecular polarizabilities thereof become higher, but when the intramolecular polarizabilities become too high, transference of electric charge occurs, and thereby the materials lose transparency. Further, the wavelength of the second harmonic generation and the maximum absorption wavelength .lambda..sub.max of the nonlinear optical organic material are coincident with each other, so that the second harmonic generation cannot be efficiently taken out.
For obtaining nonlinear optical organic materials, there have been proposed compounds in which a nitro group having high acceptability and an amino group having high donative properties are incorporated at the both ends of a linear molecule such as stilbene. However, these compounds have such problems that the adjacent molecules are apt to be centrally symmetric and the transparency is not so high.
In order to solve those problems, various methods such as a method of limiting the length of the .pi. electron conjugation system in the molecule and a method of incorporating a hetero atom into the aromatic ring have been tried, but any absolute solution to the problems has not been found out yet.
Further, there is other problem that any nonlinear optical organic material in a form of single crystal having suitable size for using as the nonlinear optical element cannot been obtained by the conventional processes. That is, almost all of the conventional nonlinear optical organic materials are extremely small sized needle crystals or single crystals having less than 1 mm.sup.3, and a nonlinear optical organic material of single crystal having 1 mm.sup.3, especially more than 5 mm.sup.3 has been hardly obtained conventionally.